1. Field of the Invention
This invention pertains to novel cement slurries which are pumpable and thixotropic. This invention also pertains to the use of such novel cement slurries in cementing pipe into a borehole of a petroleum well or other well that traverses subterranean formations. This invention also pertains to a method of sealing "thief zones" in subterranean formations.
2. Description of the Prior Art
The technology for drilling deep wells through one or more subterranean formations for the purpose of removing fluids (liquids and/or gases), particularly hydrocarbons, is well known. Normally, this entails drilling a borehole from the surface to the desired formation and inserting a casing or pipe within the borehole through which various materials may be added to stimulate well production and through which the desired liquids and/or gases are withdrawn. The borehole is of necessity of greater diameter than the casing or pipe and the space between the casing or pipe and the sides of the borehole must be filled. There are a variety of reasons why this must be done, but the most important reasons are to support and secure the pipe within the borehole and to restrict fluid movement between producing formations.
Cement slurries are normally used to fill the void between the casing or pipe and the walls of the borehole; a process called "well cementing" in the industry. In using such cement slurries, a line or string of pipe is inserted into the borehole and the cement slurry is pumped down through the pipe into the bottom of the borehole and then upwardly along the outside of the pipe until the annular space between the pipe and the borehole is filled. The cement slurry must be then cleared from the interior of the pipe before it hardens; this is normally accomplished by injecting a liquid medium behind the cement slurry and using it as a "hydraulic fluid" to force the remaining cement out of the pipe. In another but less common technique, the borehole is filled with a cement slurry and the pipe or casing (normally with the end sealed) is lowered into the hole. Cementing techniques are more fully described by D. K. Smith in "Cementing", Monograph Vol. 4, Henry L. Doherty Series, Society of Petroleum Engineers of AIME, New York (1976).
Cementing operations place very rigorous demands upon the cement slurries used. A special area of technology has been developed to supply cements capable of meeting such demands. The man of ordinary skill in the art relating to the instant invention is, therefore, a person skilled in the manufacture and use of cement slurries in well cementing.
Many patents in this area have been classified by the U.S. Patent and Trademark Office under the classification 166/285, 292 and 293.
Various additives have been developed for use in cement slurries to improve specific properties. For example, compounds have been added to: retard or accelerate the rate of cure (i.e., hydration and hardening of the slurry); prevent fluid loss; improve green strength; etc. The ideal cement slurry, of course, is one which does not change its viscosity at all until the cement is in place and then proceeds to cure very rapidly.
Various inorganic sulfate salts have been used as cement additives which normally cause the cement to expand. S. Chatterji et al., Magazine of Concrete Research, 19 (60) 185-9 (1967); J. Benstead, Cement Technology, volume 2, No. 3, 73-6 and 100 (1971); W. Gutt, et al., Cement Technology, volume 2, No. 5, 143-7 and 150-7 (1971); and Smith, ibid, at page 13. A wide variety of other references have been published which show the effect of sulfate salts on Portland cements in particular. Budnikov, Chemistry of Cement, Proceedings of the Fourth International Symposium, Natural Bureau of Standards, 469-477 (1960) is illustrative and deals with the effect of calcium sulfate hemihydrate (plaster of paris) on Portland cement.
Various inorganic chloride salts, and particularly sodium chloride and calcium chloride, have also been used as cement additives to cause expansion. See, for example, the Journal of Petroleum Technology, 187-194 (February, 1963).
In still other instances, it has been found that certain mixtures of inorganic salts have beneficial effects upon the cements. For example, Slagle (U.S. Pat. No. 3,340,929) discovered that mixtures of chloride salts (specifically sodium chloride) and sulfates (e.g., sodium sulfate) were effective in causing Portland cement to expand.
Aluminum sulfate, hydrated calcium sulfate, iron sulfate and sodium sulfate, individually, are perhaps the more common sulfates which have been added to the cement slurries. Combinations of these particular cements may have been used in the prior art, but to applicant's knowledge no one has previously described or used a combination of iron (II) sulfate and an aluminum sulfate in a cement slurry. This combination of iron (II) and aluminum sulfate salts produces a dramatic synergistic effect as will be hereafter described.